The upcoming 3rd Generation Partnership Project (3GPP) New Radio (NR) radio-access technology is based on Orthogonal Frequency Division Multiplexing (OFDM) and will support multiple numerologies in terms of subcarrier spacing, subframe (or slot) length etc. A basic subcarrier spacing f0 and a corresponding subframe design consisting of N OFDM symbols are used. Other numerologies are then achieved by scaling the basic subcarrier spacing Δf. For example, by using a subcarrier spacing of 2Δf the corresponding OFDM symbol is half as long as in the original case with Δf. The overall subframe of N OFDM symbols will consequently also be half as long as in the original case. Having the possibility for different numerologies can be beneficial in order to support different services with different requirements in terms of latency; a latency-critical service requiring low latency can use a higher subcarrier spacing and a correspondingly shorter subframe duration.
To allow for coexistence with Long Term Evolution (LTE), in particular Narrow Band Internet of Things (NB-IoT), it is beneficial to use the same subcarrier spacing f0 as in LTE and 3GPP has therefore agreed on Δf=15 kHz. Furthermore, the LTE slot/subframe structure is beneficial. In LTE, a slot consists of 7 OFDM symbols where the first OFDM symbol has a slightly longer cyclic prefix (CP) than the others. More specifically, in LTE an OFDM symbol without cyclic prefix is 2048 Ts long where Ts is the basic time unit, Ts=1/(2048×15000) seconds. The first OFDM symbol has a cyclic prefix of 160 Ts and the remaining six OFDM symbols in the slot have a cyclic prefix of 144 Ts. This OFDM symbol is shown shaded grey in FIG. 1, while the white OFDM symbols have the slightly shorter cyclic prefix.
It is beneficial if the symbol boundaries across different numerologies are time aligned as this would allow for one “long” OFDM symbol to be replaced by two (or more) “short” OFDM symbols. One usage of this is multiplexing of different services, e.g., by “replacing” one long OFDM symbol in an ongoing transmission with two (or more) short symbols for transmission of a latency-critical message. This is straightforward when moving to higher values of Δf. Each OFDM symbol in the numerology with subcarrier spacing fi (=(i+1)*Δf) is split into two symbols with subcarrier spacing fi+1 (=(i+2)*Δf) as shown in FIG. 1 too. Note that this results in the first two symbols of the 30 kHz numerology in a 0.5 ms slot having a longer cyclic prefix than the remaining 12 symbols in order to maintain the symbol boundary alignment.
It is not unlikely that subcarrier spacings lower than 15 kHz are needed, e.g., for non-latency critical machine-type communication (MTC) services or for broadcast services. One possibility to achieve this is to use the approach described above with f0 set to the lowest possible subcarrier spacing desired, e.g., 3.75 kHz. However, when this structure is scaled to 15 kHz the result would not match the LTE slot structure and as a result degrade the coexistence between NR and LTE.